The iPhone Dev Team has released the first version of the Pwnage Tool, now available at http://www.iphone-dev.org/.
The tool known as "Pwnage Tool" is currently a Mac OS X application that allows users to modify their iPhone's bootloader to allow it to boot any software. In its native state, the iPhone bootloader is what prevents it from booting unauthorized (non Apple) firmware.
We previously discussed this release which provides a new method for customizing your iPhone. As always, proceed with caution.